Modern span bridges are designed and engineered to carry heavy loads across a span extending between two sides, above an underlying feature such as a railway bed, another roadway, a river or stream, a chasm or a landscape feature such as a depression, elevation or cityscape. A typical highway bridge having a road bed for trucks, cars and other heavy vehicles, is built of one or more spans, each span extending between two massive supporting structures, such as piers or abutments. A span is formed by laying a plurality of beams, such as so-called steel I-beams, tub girders or reinforced concrete beams across the supporting structures, with the beams extending generally parallel to the road direction. Cross bracing may be employed to stabilize the beams in position. A roadway surface, such as a reinforced concrete slab surface, is then built or formed on top of the supporting beams.
The bridge beams used in each span may be large and difficult to move or maneuver. For example a beam formed of steel may be less than thirty to more than one hundred feet long, and may weigh hundreds of pounds per linear foot, with a total weight of tens of tons. Reinforced concrete beams of comparable stiffness and load bearing capacity may also be dimensionally large and of even greater weight.
Bridge construction involves site preparation and casting of concrete foundations forming abutments and piers at suitably-spaced and positioned locations, followed by positioning of the necessary bridge beams on the piers, followed by construction of the road surface. Each bridge beam must be accurately positioned across a span, onto seats or load plates on opposed abutments or piers to precisely position and define the support plane upon which the road bed is built. Moving the beams into proper positions on the support foundations is usually accomplished by attaching a beam via one or more cables to a tall heavy-load crane; extending the crane and moving the crane and/or the crane boom so that the beam lies approximately above two spaced-apart support foundations; and lowering the cable/sling while carefully guiding the ends of the beam into position on the supports. The beam size necessitates a crane having a great load capacity, especially in an extended position, in order to lift the beam over, and lower the beam down upon, the foundations or piers with the beam ends accurately positioned and aligned. Moreover, the crane boom should be quite long so that when extended to an appropriate position halfway across the span, it will lie at a steep angle and thus experience a relative low torque moment from the vertically-directed weight of the heavy beam, and thus will not tip or otherwise destabilize the crane vehicle. For example, an eighty foot or longer crane, capable of lifting thirty tons or more when extended, may be required to place even a relatively short but heavy beam.
Moving a large crane to a construction site along public roadways may require special permits, may require that the crane be disassembled and accompanied by one or more safety or escort vehicles carrying cautionary signs or lights, and may require coordination with special traffic management personnel. Care must also be taken to assure that width and weight capacity of all roads and bridges along the access route are adequate to support the crane and accommodate its spatial dimensions with clearance. The logistics for deployment of the crane can introduce substantial costs and delay into a bridge construction schedule.
In addition, larger cranes require a highly skilled operator to assure smooth and accurate operation, and they are quite specialized equipment, requiring advance scheduling. In addition, unexpected characteristics of the surrounding terrain may pose further problems for suitably stabilizing the crane vehicle on the site. All of these necessary logistic considerations and potential complications can introduce undesirable cost and delay into the construction of a bridge.
If a bridge could be constructed to the same spec without having to use a large crane for initially positioning the bridge beams, substantial efficiencies of construction and scheduling, with concomitant cost savings, could be realized and a greater degree of certainty brought to project management.